Group III nitride semiconductors offer a direct transition over a band gap energy from visible light to ultraviolet rays, and excel in the light emission efficiency, and thus have been manufactured as semiconductor light-emitting devices such as a light emitting diode (LED) and a laser diode (LD) for use in various applications. In addition, when used for an electronic device, Group III nitride semiconductors have a potential to provide electronic devices having characteristics superior to those using conventional Group III-V compound semiconductors.
Such Group III nitride compound semiconductors are, in general, produced from trimethyl gallium, trimethyl aluminum, and ammonia as raw materials through a Metal Organic Chemical Vapor Deposition (MOCVD) method. The MOCVD method is a method in which a vapor of a raw material is introduced into a carrier gas to convey the vapor to the surface of a substrate and decompose the raw material on the surface of the heated substrate, to thereby grow a crystal.
Hitherto, a single crystalline wafer of a Group III nitride semiconductor has not been commercially available, and Group III nitride semiconductors are, in general, produced by growing a crystal on a single crystalline wafer of a different material. There is a large lattice mismatching between such a different kind of substrate and a Group III nitride semiconductor crystal to be epitaxially grown thereon. For example, when gallium nitride (GaN) is grown directly on a substrate made of sapphire (Al2O3), there is a lattice mismatching of 16% therebetween. Also, when gallium nitride is directly grown on a substrate made of SiC, there is a lattice mismatching of 6% therebetween. In general, a large lattice mismatching as in the above leads to a problem in that it is difficult to epitaxially grow a crystal directly on a substrate, or a crystal, even if grown, can not gain excellent crystallinity.
Thus, for epitaxially growing a Group III nitride semiconductor crystal on a single crystalline sapphire substrate or a single crystalline SiC substrate through a Metal Organic Chemical Vapor Deposition (MOCVD) method, a method has been proposed and generally performed in which, firstly, a layer called a low temperature buffer layer made of aluminum nitride (AlN) or aluminum nitride gallium (AlGaN) is laminated on a substrate, and then a Group III nitride semiconductor crystal is epitaxially grown thereon at a high temperature (for example, Patent Documents 1 and 2).
However, in the methods disclosed in the Patent Documents 1 and 2, lattice match is not basically achieved between a substrate and a Group III nitride semiconductor crystal grown thereon. Therefore, the state is prepared in which a dislocation called a threading dislocation that extends toward a surface is included in a grown crystal. As a result, distortion occurs in a crystal, and sufficient light emission strength cannot be obtained unless the structure is appropriately adjusted. In addition, there was a problem such as a deterioration of productivity.
In addition, a method has been proposed in which the aforementioned buffer layer is formed by a method other than the MOCVD method, and for example, a method has been proposed in which a buffer layer is formed by high frequency sputtering, and a crystal having the same composition is grown thereon by an MOCVD method (for example, Patent Document 3). However, the method disclosed in Patent Document 3 has a problem in that an excellent crystal cannot be stably laminated on a substrate.
Thus, in order to stably produce an excellent crystal, there have been proposed a method for annealing a buffer layer in a mixed gas made of ammonia and hydrogen on completion of its growth (for example, Patent Document 4), and a method for forming a buffer layer by DC sputtering at a temperature of 400° C. or higher (for example, patent document 5). However, in any of the methods disclosed in the Patent Documents 4 and 5, there was the problem that it was difficult to stably obtain a good crystal when lattice mismatch occurred between a substrate and a Group III nitride semiconductor crystal grown thereon.
In addition, in any of the Patent Documents 1 to 5, there was the big problem that the crystallinity of the (10-10) plane of the ground layer made of GaN, which strongly relates to the dislocation density of a crystal and has an important role to improve properties of the light-emitting device such as an LED, was particularly poor.
[Patent Document 1]
Japanese Patent No. 3026087
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. Hei 4-297023
[Patent Document 3]
Japanese Examined Patent Application, Second Publication No. Hei 5-86646
[Patent Document 4]
Japanese Patent No. 3440873
[Patent Document 5]
Japanese Patent No. 3700492